1. Field of the Invention
The present invention relates to a ball screw using for a feed mechanism disposed in various machines.
2. Description of the Related Art
In a ball screw of this type, a nut is fitted with an outer periphery of a screw shaft. Helical ball rolling grooves are formed in an outer peripheral surface of the screw shaft and the inner peripheral surface of the nut. The mutually opposed ball rolling grooves cooperate together in forming a ball rolling path between the ball rolling grooves.
A return path is formed in the nut. The return path connects together one end portion of the ball rolling path and the other end portion thereof.
Both ball rolling path and return path cooperate together to form an endless circulation path. A plurality of load balls are mounted within the endless circulation path in such a manner that they are adjacently arranged one another. Accordingly, the screw shaft and nut are allowed to make smooth helical motion with respect to each other through the rolling motion of the load balls, which are in the ball rolling path.
That is, in accordance with the relative helical motion between the screw shaft and the nut, the load balls circulate endlessly while they are rolling in the endless circulation path when the load balls roll along a part of the ball rolling path, which is formed by the ball rolling grooves, loads for power transmission are applied to the load balls.
The return path, which connects together the two end portions of the ball rolling path formed by the ball rolling grooves, is formed of e.g. a metal-made tube This tube is disposed on the nut in such a manner that a portion thereof is exposed to the outer peripheral portion of the nut.
The position of the tube disposed on the nut (the position of the tube with the center axis of the nut as the standard) changes variously according to both setting state of the ball screw and an operation of the nut. The position of the tube can have an influence on a dynamic torque characteristic of the ball screw in connection with gravitation.
A large number of load balls with a mounting clearance are incorporated within the endless circulation path of the ball screw.
Within the endless circulation path of the ball screw, there are incorporated a large number of load balls with a mounting clearance between the load balls.
That is, where a whole block length of the endless circulation path is expressed as L, the diameter of the load ball is expressed as D and the number of the load balls is expressed as n, there is generated a mounting clearance of L−(D×n).
Due to the mounting clearance, especially when the ball screw operates or rotates at a low speed, the load balls are influenced by gravitation depending on the position relationship of the tube. The gravitation does not have a good effect on the operation characteristic (dynamic torque characteristic) of the ball screw.
In case where the low-speed operating tube is disposed so as to face upwardly, the above mounting clearance occurs within the tube. This causes the load balls to rub against each other, so that a clogging phenomenon occurs.
FIG. 7 shows a structure of a connecting portion between a ball rolling path 5 and a tube 6 as a return path. The ball rolling path 5 are formed by both ball rolling groove 2 in a screw shaft 1 and ball rolling groove 4 in a nut 3.
In this structure, a load ball 7 is moved in accordance with a relative helical rotation of the nut 3 with respect to the screw shaft 1 in such a manner that it is scooped up into the tube 6.
The load ball scoop-up portion, in which the ball rolling path 5 and tube 6 are connected together, has a clearance in the diameter direction of the load balls with respect to the diameter of the load ball and thus play between the load ball scoop-up portion and load ball 7 are in a certain degree.
Therefore, when the load balls 7 pass through the load ball scoop-up portion, they are arranged side by side, that is, there occurs a side-by-side arrangement phenomenon.
The side-by-side arrangement varies depending on the position relationship of the tube 6, so that the side-by-side arrangement does not have a good effect on the operation characteristic of the ball screw.
FIG. 8A shows a dynamic torque characteristic of a ball screw when the ball screw is installed horizontally and a tube is disposed upwardly of the nut. FIG. 8B shows a dynamic torque characteristic of the ball screw when the tube is disposed downwardly of the nut.
That is, FIG. 8A shows the dynamic torque characteristic of the ball screw when the tube is disposed upwardly of the nut, and FIG. 8B shows the dynamic torque characteristic of the ball screw when the tube is disposed downwardly of the nut.
When the tube is disposed upwardly of the nut, the torque is varied due to the load ball clogging phenomenon.
When the tube is disposed downwardly of the nut, a slight torque variation which is referred to as a so called “mustache”.
FIGS. 8(A), (B) respectively show especially the dynamic torque characteristics of the ball screw in the low-speed rotation in which the operation characteristics of the ball screw appears outstandingly as the torque characteristics thereof.
In case where the tube of the ball screw is disposed at a specific position where the tube has no effect on the operation characteristic of the ball screw, there arises almost no problem.
However, for example, when the ball screw is used in such a manner that a screw shaft thereof is fixed and a nut disposed on the outer periphery of the screw shaft is helically rotated with respect to the screw shaft, the position of the tube gradually varies according to the helical rotation of the nut. Accordingly, it is impossible to determine the position of the tube at a specific position.
In case where a ball screw includes a plurality of endless circulation paths and a plurality of tubes, as a return paths which are corresponding to endless circulation paths, and which are disposed on the outer periphery of a nut in a state that tubes are shifted in phase from one another, even when the position of one of the tubes is determined at a specific position, the positions of the remaining tubes are shifted out of the specific position.
Thus, in the case of a ball screw that position of the return path cannot be limited to a single specific position, the position relationship of the tubes cause to an ill influence on the operation characteristic of the ball screw.
The operation characteristic of the ball screw is worsened, since the balls 7 are directly contacted with each other to thereby rub against each other as shown in FIG. 9.
The mutually rubbing state varies according to the position of the tube. The mutually rubbing actions of the load balls 7 cause the load balls 7 to wear and also damage them, that is, the mutually rubbing actions of the load balls 7 have ill influences on the operation characteristic of the ball screw in these respects as well.
The above-mentioned data on the torque characteristic relate to a ball screw which is a pre-loaded product. For example, even in the case of a clearance screw, in a state where a load is applied to the clearance screw, the relationship between the load balls is similar to the pre-loaded products. Therefore, preferably, there may be eliminated the mutually rubbing actions between the load balls.
Conventionally, as measures against the worsened operation characteristic of the ball screw, as shown in FIG. 10, a spacer ball 8 having a diameter smaller by several tens of μm than the load ball 7 is inserted between the mutually adjoining load balls 7 to thereby avoid the mutual contact between the load balls 7.
However, in this case, the number of load balls 7, which can be mounted into the endless circulation path, is reduced down to ½ or ⅓. Therefore, the ball screw of this type has disadvantages in the load capacity and rigidity thereof.